1. Field of the Invention
The present invention relates to a pilot control valve that achieves a continuous and consistent pumping rate for a reciprocating pump. More particularly, the pilot control valve of the present invention relates to a pilot control valve that controls the flow of control fluid to a piston, valve or the like to drive a reciprocating device such as a chemical or glycol injection pump. The pilot control valve of the present invention controls such flow through a pneumatic valve mechanism having a movable valve member and a plurality of slide valves slideably engaging the movable valve member. By selectively communicating and venting pressurized control fluid through a plurality of control fluid conduits, the pilot control valve of the present invention provides for increased pressurization and venting of the control fluid acting on the piston to increase the pumping speed of the reciprocating device.
2. General Background
There are various prior art devices known for controlling reciprocating pumps. Many prior art devices use a mechanical control mechanism to drive the piston of the reciprocating pump, but these mechanisms have been unreliable either because they require a number of failure- and/or wear-prone components or because they can stall or vary in stroke frequency in response to varying operating conditions frequently encountered in practical usage. One pilot control valve invention using pneumatic valve control and improving on these prior art devices is the pilot control valve invention disclosed in U.S. Pat. No. 6,183,217 B1, entitled xe2x80x9cPilot Control Valve for Controlling a Reciprocating Pumpxe2x80x9d which issued on Feb. 6, 2001.
The pilot control valve disclosed in U.S. Pat. No. 6,183,217 B1 changes the directional flow of control fluid to a piston coupled to the pilot control valve to drive a reciprocating device. More specifically, the pilot control valve of U.S. Pat. No. 6,183,217 B1 includes a valve member shiftable within a valve body between a first or xe2x80x9cdownstrokexe2x80x9d position and a second or xe2x80x9cupstrokexe2x80x9d position. When in its first position, the valve member allows communication of pressurized control fluid supplied to the valve body to the lower surface of the piston to initiate movement of the piston from its first or xe2x80x9cdownstrokexe2x80x9d position to its second or xe2x80x9cupstrokexe2x80x9d position. The pressurized control fluid is communicated to the lower surface of the piston through a first pressurized fluid conduit extending along the length of the valve body outside of the valve body. As the piston reaches its second position, a vent in a rod attached to the piston allows control fluid acting on the valve member retaining the valve member in its first position to depressurize and vent from the valve body. The pressurized control fluid is vented from the valve body through a fluid exhaust conduit extending out of the valve body. As such control fluid is depressurized and vented, pressurized control fluid acts on the valve member to initiate movement of the valve member from its first position to its second position. As the valve member moves from its first position to its second position, a slide valve portion of the valve member advances with the valve member from a first position to a second position. In its second position, the valve member through the positioning of the slide valve portion precludes communication of control fluid to the lower surface of the piston and allows communication of pressurized control fluid to the upper surface of the piston causing the piston to return to its first position. The pressurized control fluid is communicated to the upper surface of the piston through a second pressurized fluid conduit extending along the length of the valve body but within the valve body. As the piston returns to its first position, the vent in the piston rod allows the pressurized control fluid acting on the upper surface of the piston to act on the valve member to move the valve member back to its first position. As the valve member returns to its first position, the slide valve portion of the valve member also returns to its first position. In its first position, the valve member through the positioning of the slide valve portion precludes communication of the control fluid to the upper surface of the piston and allows the pressurized control fluid to vent through a fluid exhaust conduit. The valve member through the positioning of the slide valve portion also allows communication of the control fluid through the first pressurized fluid conduit to the lower surface of the piston and the process is repeated over and over. The duration of each cycle is varied by adjusting a backpressure valve that varies the rate that the control fluid acting on the piston is depressurized and vented from the valve body during each cycle. This process is repeated over and over to achieve a consistent pumping rate for the reciprocating device that uses only pneumatic valve control.
The pilot control valve of U.S. Pat. No. 6,183,217 B1 overcame the prior art devices by improving reliability by controlling the communication of control fluid to a piston included with a reciprocating device using pneumatic valve control rather than a mechanical control mechanism. Although the pilot control valve disclosed in U.S. Pat. No. 6,183,217 B1 has significant advantages, there is still a need for a pilot control valve that delivers a greater volume of control fluid at an increased pressure to drive reciprocating devices at increased stroke rates. Furthermore, there is a need for a pilot control valve that can be tuned to prevent stalling under differing pressure, viscosity, and/or compressibility properties of the control fluid. Finally, there is a need to reduce the operating impact stresses on the pilot control valve by more smoothly transitioning the valve member from its first position through its second position. Such improved performance would need to be achieved without sacrificing reliability and by still providing for the complete control of the piston in a pneumatic manner.
The pilot control valve of the present invention represents an improvement over the pilot control valve of U.S. Pat. No. 6,183,217 B1 for most reciprocating device applications because it increases the stroke rate of the reciprocating device, it prevents stalling, it increases the tolerance of the reciprocating device to varying properties of the control fluid, it reduces the likelihood of freezing of the control fluid, and it reduces the impact stresses on the valve member, but still relies solely on pneumatic valve control. These improvements are realized with an actual increase in reliability.
Similar to the pilot control valve of U.S. Pat. No. 6,183,217 B1, the pilot control valve of the present invention is positioned above the piston included with the reciprocating device to provide linear, reciprocating force using compressible or non-compressible pressurized control fluid to drive the piston. The pilot control valve of the present invention controls the communication of the control fluid to the piston using pneumatic valve control.
More specifically, the pilot control valve of the present invention includes a valve member shiftable within a valve body between a first or xe2x80x9cdownstrokexe2x80x9d position and a second or xe2x80x9cupstrokexe2x80x9d position. When in its first position, a pair of slide valves slideably engaging the valve member allow communication of control fluid supplied to the valve body to the lower surface of the piston to initiate movement of the piston from its first position to its second position. Simultaneously, the slide valves allow the control fluid acting on the upper surface of the piston to vent through exhaust ports located in the valve body. The pressurized control fluid is communicated to the lower surface of the piston through a first pair of pressurized fluid conduits extending along the length of the valve body. The pilot control valve of U.S. Pat. No. 6,183,217 B1 delivered pressurized control fluid to the upper surface of the piston through a single fluid conduit. By using a pair of pressurized fluid conduits, two distinct advantages are gained. First, an offset of variable magnitude can be introduced between the two slide valves, which enables the pilot control valve to operate without stalling under varying properties of the control fluid and reduces impact stress on the valve member. Second, the volume of control fluid delivered to the piston is doubled, increasing the maximum pumping speed significantly and preventing freezing of wet control fluid (such as humid compressed air or natural gas) which is prevalent with the single fluid conduit and port design disclosed in U.S. Pat. No. 6,183,217 B1 and with competitive designs. While this second advantage could also be achieved by increasing the size of the single port fluid conduit and port, increasing the fluid conduit and port size would increase the size of the entire mechanism whereas no size increase is required to add a second slide valve and fluid conduit.
As the piston reaches its second position, a poppet in a piston rod attached to the piston is in a xe2x80x9cclosed,xe2x80x9d or first position, allowing control fluid acting on the valve member retaining the valve member in its first position to depressurize and vent from the valve body through a hole located just above the poppet. The pressurized control fluid is vented from the valve body through a fluid exhaust conduit extending out of the valve body. As such control fluid is depressurized and vented, pressurized control fluid acts on the valve member to initiate movement of the valve member from its first position to its second position. As the valve member moves to its second position, the valve member advances the slide valves upward from a first position to a second position.
In its second position, the valve member through the positioning of the slide valves precludes communication of control fluid to the lower surface of the piston and allows communication of pressurized control fluid to the upper surface of the piston causing the piston to return to its first position. The slide valves simultaneously allow communication of the control fluid acting on the lower surface of the piston to exhaust through ports located in the valve body. The two ports providing the exhaust of the control fluid together provide for a lower pressure drop of the control fluid as it vents from the lower surface of the piston, decreasing the temperature drop, and thereby reducing the risk of freezing. Pressurized control fluid is communicated to the upper surface of the piston using a second pair of pressurized fluid conduits rather than a single fluid conduit as described in U.S. Pat. No. 6,183,217 B1. Both pressurized fluid conduits extend downward through the valve body to the piston to deliver control fluid to act on the upper surface of the piston. In this way, an increased volume of control fluid acts on the piston to increase its speed.
As the piston returns to its first position, the lower hole in the piston rod becomes exposed to the pressurized control fluid acting on the upper surface of the piston. The poppet in the piston rod moves to its xe2x80x9copenxe2x80x9d or second position as the pressurized control fluid acting on the upper surface of the piston acts on the valve member to move the valve member back to its first position. As the valve member returns to its first position, the valve member advances the slide valves downward from a second position to a first position. In its first position, the valve member through the positioning of the slide valves precludes communication of the control fluid to the upper surface of the piston and simultaneously allows the pressurized control fluid to vent through a fluid exhaust conduit. The valve member through the positioning of the slide valves also allows communication of the control fluid through the first pair of pressurized fluid conduits to the lower surface of the piston and the cycle is repeated. At this moment, and prior to the initial upward motion of the piston, the control fluid holding the valve member in its first position may under various pressure, viscosity, and/or compressibility properties of the control fluid begin to flow in reverse along the same path it followed when pressurizing the valve member to move the valve member to its first position. This tendency is especially severe in the case that the control fluid is either a mixture of liquid and gas phases, or when the pressure of the control fluid is especially high. In the invention as described in U.S. Pat. No. 6,183,217 B1, this flow would have the effect of causing the valve member to move back toward its second position, possibly causing a stall condition. In the present invention, this backward flow causes the poppet to move to its xe2x80x9cclosedxe2x80x9d or first position, blocking any backward flow and preventing movement of the valve member from the first position until the piston begins to move back towards its second position and the lower hole in the piston rod is once again isolated from the control fluid acting on the piston. The duration of each cycle can be varied by adjusting a backpressure valve that varies the rate that the control fluid acting on the piston is depressurized and vented from the valve body during each cycle. This process is repeated over and over to achieve a consistent pumping rate for the reciprocating device that uses only pneumatic valve control.
As noted, the pilot control valve of the present invention increases the stroke rate of the reciprocating device by increasing the volume of the control fluid delivered to the piston surfaces during each stroke. This increase in volume is achieved using the dual pair of pressurized fluid conduits to communicate fluid from the valve body to the piston chambers. The first pair of pressurized fluid conduits communicates control fluid from the valve body to the lower surface of this piston to urge the piston to its second position. The second pair of pressurized fluid conduits communicates control fluid from the valve body to the upper surface of the piston to urge the piston from its second position back to its first position. The pilot control valve of the present invention delivers such pressurized control fluid and achieves such increased stroke rate with improved reliability.
The pilot control valve of the present invention also eliminates the risk of stalling of the valve member during each stroke cycle because the pair of slide valves can be offset relative to one another. The magnitude of the offset depends upon the properties of the control fluid. This offset allows movement of one slide valve to be initiated at a different position of the valve member than the movement of the second slide valve. As the valve member moves upward, the lower edge of the first valve slide is engaged by the valve member just prior to the lower edge of second slide valve being engaged by the valve member. The invention as described in U.S. Pat. No. 6,183,217 B1 was susceptible to stalling when the single slide valve could simultaneously block both the upper and lower ports, preventing movement of the piston to either the first or second position. The present invention eliminates this possibility by guaranteeing that at least one port will remain partially open at all times, thus guaranteeing movement of the piston to either the first or the second positions. The offset of the present invention has the further effect of rounding the response of the valve member at the points where the valve member changes its direction of movement rather than having abrupt changes in movement, reducing impact stress on the valve member and thus extending its life dramatically. The invention as described in U.S. Pat. No. 6,183,217 B1 created the need for the valve member to be made of a highly impact-resistant material, increasing its cost, while the present invention eliminates that cost. The magnitude of the offset can be as low as zero (no offset) depending on the particular properties of the control fluid.
These and other features and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings and the appended claims.